1. Field of the Invention
The present invention relates to a power supply monitor which monitors a drop of a voltage supplied by a power supply in an apparatus such as a memory card which needs back-up of data.
2. Description of the Prior Art
An apparatus such as a SRAM (static random access memory) card holding data in a volatile memory needs a power supply monitor which generates a reset signal for inactivating the memory or a control IC therefor when the power supply voltage decreases below a certain voltage.
In a previous power supply monitor, a comparator compares a power supply voltage with a threshold voltage, and it generates a reset signal when the power supply voltage decreases lower than the threshold voltage. The comparator is required to respond the drop in voltage without delay because the reset signal has to be supplied before the power supply voltage decreases but still serves back-up of data. However, if the comparator responds with no delay, the reset signal is generated even for an instantaneous voltage drop, for example, when a noise superposes the power supply voltage, though such instantaneous voltage drop does not affect the data stored in the memory. If the power supply monitor is used under a bad environment having much noises, such a system including the power supply monitor is practically always reset, and this is very inconvenient. Therefore, if a system such as a SRAM card has a power supply monitor, the system itself has to prepare a countermeasure, for example, by removing noises.